1. Field of the Invention
The present invention relates to a semiconductor device, in particular, a semiconductor device using a non-single-crystal semiconductor layer in a light-receiving element.
In this specification, a semiconductor device refers to a device which can function by utilizing semiconductor characteristics, and an electro-optical device, a semiconductor circuit, and an electronic device are all included in the category of the semiconductor device.
2. Description of the Related Art
Various kinds of optical touch sensors have been recently proposed; in the optical touch sensor, a photosensor circuit is incorporated into a display portion of a display device. In general, one photosensor circuit is provided for a unit pixel or three subpixels (RGB). The photosensor circuit includes a light-receiving portion consisting of a light-receiving element and the like, and an amplifier circuit portion which amplifies a signal received from the light-receiving element and outputs a detected signal.
In the case where a photosensor circuit 11 is formed using a limited number of masks, that is, in the case where the photosensor circuit 11 is formed on the same plane as an amplifier circuit portion 12, the amplifier circuit portion 12 occupies a smaller area in the photosensor circuit 11 than a light-receiving portion 13. Thus, the difference in area causes a dead space 14 in a display portion as illustrated in FIG. 8A.
When the area of the light-receiving portion 13 is increased in order to increase the photosensitivity of the photosensor circuit 11, an increase in the area of the photosensor circuit 11 in the display portion further proceeds.
In Patent Document 1, the area of a unit pixel 15 is reduced by half of the area of the dead space 14 in order to remove the dead space 14 caused by a difference in the area between the amplifier circuit portion 12 and the light-receiving portion 13 in the photosensor circuit 11 (see FIG. 8B).
In Patent Document 2, a photodiode is used for a semiconductor layer in a light-receiving element. The photodiode has a stacked structure of a p-layer of microcrystalline silicon, an i-layer of amorphous silicon, and an n-layer of microcrystalline silicon.